I think the reason GPLv3 isn't tackling trademarks is that they haven't
become a big problem, and there aren't clear signs that they will.

That's good then. It seemed possible trademarks could be used to exert
some "control".. in the same sense that a software license ensuring
source code is useless if the code is all covered by software parents
which are enforceable. A clause would prevent trademarks being used in
the same way, It'd be a shame to have to say "I told you so" in the
future.
Imagine if a toolkit vendor said all the "Zt" had to be removed, and
all the classes have the trademark "Z" name prefix removed on anyone's
redistributed copies. It might be practically not worth the effort in
attempting the task!
Kind regards, Jon